You can do a LOT more with a decent Interface Library than AFK checking. Think about the type of information that needs to be displayed in your game and think of all the possible ways you could display it to the player and which methods are more effective for what kind of data.

Of course, a simple "You have been marked as AFK." would work, and would make the system of AFK's look much cleaner.
After thinking it could also be used for, maybe world reboots(and anything about the world obby)? How could you ignore that window.

Either way the uses are as endless as long as you have information that usually pops-up on screen or is posted in an output somewhere.